1. Field of the Invention
This invention relates to an electronic circuit and more particularly to a circuit for boosting a voltage to a regulated value and for maintaining the regulated value over a wide variety of load demands.
2. Background of the Relevant Art
Amplifiers for boosting power or voltage are well known. Power amplifiers are designed to provide large current to the output load. The load impedance must be low enough to allow a high-current output, but not so low that the signal is distorted excessively. Voltage amplifiers increase the voltage level of an applied signal but do not necessarily increase the power to the output load. Both power amplifiers and voltage amplifiers can be achieved using various circuit configurations. For example, amplification can be performed using electron tube amplifiers or solid-state transistors such as bipolar or MOS transistors.
A problem associated with many amplification circuits is the relatively large amount of power required to drive the circuits. In particular, power amplifiers require a large amount of output current and therefore cannot, in most cases, be used in low-power applications. Likewise, voltage amplifiers often utilize op-amps or bipolar transistors which consume a large amount of current when active. Still further, in order to provide amplification, the amplifier circuit is generally maintained in an "on" condition throughout its operation. Thus, the amplifier circuit consumes power the moment it is turned on until it is subsequently turned off.
In many applications, it is important that voltage amplification be achieved with minimum power consumption. Examples of low-power applications include battery-operated devices, one of which is a portable computer. A portable computer can operate in various modes of operation, including a standby mode and a full power mode. During standby operations, power drain upon the computer's battery or AC wall unit is minimized. During full power operation (i.e., whenever the display is on, CPU is operating at full speed, peripheral devices are on, resume switch is on, and/or a modem ring is detected), power drain is not minimized. The computer is selectively placed in the standby or full power mode whenever respective power switching transistors which drive the computer are turned off or on by the power management logic. It is important, however, that whenever a selection is made, the power transistors which are active are fully placed in full saturation region. If the power transistors which are coupled between the power source and the computer load are not placed fully within saturation, then the "on" resistance of the transistors will consume an unacceptable amount of power.
Selectively activating the power transistors in order to choose a mode of operation is important to portable computer functionality. It is not only important that the large power transistors which provide power management switching of the portable computer be carefully monitored in order to ensure their on resistance is minimized, but is also important that select power transistors when chosen to be off are fully off. If the transistors remain slightly on, then deleterious leakage current will exists causing a waste of power.
The amplifying circuit is thereby necessary for providing amplified voltages to the gates of the power transistors (n-channel MOSFETs) which selectively modulate power (standby, full, etc.) to the portable computer. The output voltage from the amplifier to the power transistor's gate must be higher than the input voltages which powers the computer--the input voltages being coupled to both the amplifier circuit and the power transistor's source or collector--in order to ensure the transistors are fully driven into the saturation region when selected. Although the amplified output voltage must be larger than the input voltage, it is important that the power consumed by the amplifier necessary to maintain the amplified output to the power transistors (or load) be minimized.